Method for model based exhaust mixing control in a fuel cell application

ABSTRACT

A system and method for controlling a bleed valve and a compressor in a fuel cell system during an anode exhaust gas bleed so as to maintain the concentration of hydrogen within a mixed cathode exhaust gas and anode gas below a predetermined percentage. The system uses a valve orifice model to calculate the flow rate of the anode exhaust gas through the bleed valve to identify how much airflow from the compressor is required to dilute the hydrogen in the anode gas to be below the predetermined percentage. The system also uses sensor inaccuracies and production tolerances in the valve orifice model to ensure that the concentration of hydrogen in the mixed anode and cathode exhaust gas is below the determined percentage.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a system and method for controllinga nitrogen bleed from an anode sub-system in a fuel cell system and,more particularly, to a system and method for controlling a nitrogenbleed from an anode sub-system in a fuel cell system, where the methodincludes mixing the anode bleed gas with a cathode exhaust gas, andcontrolling the cathode input air based on the concentration of hydrogenin the anode bleed gas so as to maintain the concentration of hydrogenin the combined cathode and anode exhaust gas below a certainpercentage.

2. Discussion of the Related Art

Hydrogen is a very attractive fuel because it is clean and can be usedto efficiently produce electricity in a fuel cell. A hydrogen fuel cellis an electrochemical device that includes an anode and a cathode withan electrolyte therebetween. The anode receives hydrogen gas and thecathode receives oxygen or air. The hydrogen gas is dissociated in theanode to generate free protons and electrons. The protons pass throughthe electrolyte to the cathode. The protons react with the oxygen andthe electrons in the cathode to generate water. The electrons from theanode cannot pass through the electrolyte, and thus are directed througha load to perform work before being sent to the cathode.

Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell forvehicles. The PEMFC generally includes a solid polymer electrolyteproton conducting membrane, such as a perfluorosulfonic acid membrane.The anode and cathode typically include finely divided catalyticparticles, usually platinum (Pt), supported on carbon particles andmixed with an ionomer. The catalytic mixture is deposited on opposingsides of the membrane. The combination of the anode catalytic mixture,the cathode catalytic mixture and the membrane define a membraneelectrode assembly (MEA). MEAs are relatively expensive to manufactureand require certain conditions for effective operation.

Several fuel cells are typically combined in a fuel cell stack togenerate the desired power. For example, a typical fuel cell stack for avehicle may have two hundred or more stacked fuel cells. The fuel cellstack receives a cathode input reactant gas, typically a flow of airforced through the stack by a compressor. Not all of the oxygen isconsumed by the stack and some of the air is output as a cathode exhaustgas that may include water as a stack by-product. The fuel cell stackalso receives an anode hydrogen reactant gas that flows into the anodeside of the stack. The stack also includes flow channels through which acooling fluid flows.

The fuel cell stack includes a series of bipolar plates positionedbetween the several MEAs in the stack, where the bipolar plates and theMEAs are positioned between two end plates. The bipolar plates includean anode side and a cathode side for adjacent fuel cells in the stack.Anode gas flow channels are provided on the anode side of the bipolarplates that allow the anode reactant gas to flow to the respective MEA.Cathode gas flow channels are provided on the cathode side of thebipolar plates that allow the cathode reactant gas to flow to therespective MEA. One end plate includes anode gas flow channels, and theother end plate includes cathode gas flow channels. The bipolar platesand end plates are made of a conductive material, such as stainlesssteel or a conductive composite. The end plates conduct the electricitygenerated by the fuel cells out of the stack. The bipolar plates alsoinclude flow channels through which a cooling fluid flows.

The MEAs are porous and thus allow nitrogen in the air from the cathodeside of the stack to permeate therethrough and collect in the anode sideof the stack, referred to in the industry as nitrogen cross-over.Nitrogen in the anode side of the fuel cell stack dilutes the hydrogensuch that if the nitrogen concentration increases beyond a certainpercentage, such as 50%, the fuel cell stack becomes unstable and mayfail. It is known in the art to provide a bleed valve at the anode gasoutput of the fuel cell stack to remove nitrogen from the anode side ofthe stack.

The gas that is periodically bled from the anode side typically includesa considerable amount of hydrogen. Because the hydrogen will mix withair if it is vented to be in the environment, a potential combustiblemixture may occur which provides obvious safety concerns. It is known inthe art to direct the bled gas to a combustor to burn most or all of thehydrogen therein before the bled gas is exhausted to the environment.However, the combustor adds a significant cost and complexity to thefuel cell system, which is undesirable.

It is also known in the art to eliminate the combustor and directly mixthe anode bleed gas with the cathode exhaust gas. If the anode bleed gasis directly mixed with the cathode exhaust gas without control, theamount of hydrogen in the anode exhaust gas is unknown. A hydrogenconcentration sensor can be provided in the cathode exhaust gas lineafter the mixing point with the anode bleed gas to detect theconcentration of hydrogen. The hydrogen concentration sensor wouldprovide a signal to the controller during the bleed indicative of theconcentration of hydrogen in the mixed exhaust gas. If the concentrationof hydrogen was to high, the controller would increase the speed of thecompressor to provide more cathode exhaust air to lower theconcentration of hydrogen. If the compressor was unable to effectivelykeep the concentration of hydrogen below the safe limit for the stackload, then the controller would have to close the bleed valve. However,the hydrogen sensor would have to be inexpensive and be able towithstand the humidity of the exhaust gas. Currently, known hydrogenconcentration sensors are unable to fulfill these requirements.

SUMMARY OF THE INVENTION

In accordance with the teachings of the present invention, a system andmethod are disclosed for controlling a bleed valve and a compressor in afuel cell system during an anode bleed so as to maintain theconcentration of hydrogen within a mixed cathode exhaust gas and anodebleed gas below a predetermined percentage. The system uses a valveorifice model to calculate the flow rate of the anode bleed gas throughthe bleed valve to identify how much airflow from the compressor isrequired to dilute the hydrogen in the mixed gas to be below thepredetermined percentage. The system also takes sensor inaccuracies andproduction tolerances into account to ensure that the concentration ofhydrogen in the mixed anode and cathode exhaust gas is below thedetermined percentage.

Additional features of the present invention will become apparent fromthe following description and appended claims, taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a fuel cell system employing a techniquefor controlling an anode bleed, according to an embodiment of thepresent invention; and

FIG. 2 is a block diagram of a control scheme for controlling a bleedvalve in the system shown in FIG. 1.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following discussion of the embodiments of the invention directed toa system and method for controlling an anode bleed in a fuel cell systemis merely exemplary in nature, and is in no way intended to limit theinvention or its applications or uses.

FIG. 1 is a schematic block diagram of a fuel cell system 10 including afuel cell stack 12. The fuel cell system 10 is intended to generallyrepresent any type of fuel cell system that requires an anode exhaustgas bleed to remove nitrogen from the anode side of the stack 12, suchas fuel cell systems that recirculate the anode exhaust gas back to theanode inlet and fuel cell systems that employ a split stack design withanode flow shifting. Hydrogen gas from a hydrogen source 14 is providedto the anode side of the fuel cell stack 12 on line 18. An anode exhaustgas is output from the fuel cell stack 12 on line 20 and is sent to ableed valve 26. A cathode exhaust gas from the stack 12 is output fromthe stack 12 on cathode exhaust gas line 34.

As discussed above, nitrogen cross-over from the cathode side of thefuel cell stack 12 dilutes the hydrogen in the anode side that affectsstack performance. Therefore, it is necessary to periodically bleed theanode exhaust gas to reduce the amount of nitrogen in the anodesub-system. In this embodiment, the bled gas in the line 28 is mixedwith the cathode exhaust gas on line 34 in a mixing junction 36.

In order to monitor the anode sub-system, various sensors are providedin the system 10. Particularly, a pressure sensor 40 measures thepressure at the inlet to the bleed valve 26, a pressure sensor 42measures the delta pressure across the bleed valve 26 and a temperaturesensor 44 measures the temperature of the anode exhaust gas at the inletto the bleed valve 26. The pressure sensor 40 can be any pressure sensorthat measures the pressure of the anode sub-system, and a stack coolanttemperature sensor can be used instead of the temperature sensor 44.Further, a flow meter 46 measures the flow of air being input to thecathode side of the fuel cell stack 12. In an alternate embodiment, theflow meter can be eliminated and the flow rate of the compressor air canbe derived based on various factors, such as a compressor map,compressor speed, inlet/outlet pressure, temperature, etc.

As discussed above, it is necessary to control the bleed of the anodeexhaust gas to the cathode exhaust gas line 34 so that the concentrationof hydrogen therein is maintained below a predetermined safe level.Typically, it is desirable to maintain the percentage of hydrogen in themixed anode and cathode exhaust gas to be less than a few percent byvolume. In order to perform this function, a controller 48 receives thetemperature signal from the temperature sensor 44, the pressure signalfrom the pressure sensor 40, the pressure signal from the pressuresensor 42 and the flow signal from the flow meter 46. The controller 48includes an algorithm, discussed below, that determines theconcentration and the amount of hydrogen being bled from the bleed valve26, and controls the compressor 30 and the bleed valve 26 to maintainthe concentration of hydrogen in the combined exhaust gas below apredetermined level.

The algorithm calculates the concentration of hydrogen that is beingvented to the atmosphere. This concentration of hydrogen is based on thecathode exhaust gas flow and the anode exhaust gas flow. The cathode gasflow is provided by the flow meter 46. According to one embodiment ofthe present invention, the anode exhaust gas flow is calculated based onan orifice model of the bleed valve 26. The actual mole fractions ofnitrogen, hydrogen and water vapor in the anode exhaust gas iscalculated based on the assumption that the water fraction is about 100%relative humidity for the measured temperature. The dry hydrogen molefraction can be estimated by evaluating cell voltages, making use ofspecific sensors or setting the hydrogen mole fraction to 1 as a worstcase assumption.

FIG. 2 is a block diagram of a system 60 for controlling the bleed valve26 and the compressor 30 during an anode exhaust gas bleed, according toan embodiment of the present invention. The system 60 includes a stackcontrol module 62 that generates a bleed request signal on line 66 atthose times it is necessary for the bleed valve 26 to be opened toreduce the amount of nitrogen in the anode exhaust gas. Varioustechniques for determining when to bleed the anode exhaust gas are knownin the art, some of which model the concentration of nitrogen in theanode exhaust gas. For example, U.S. patent application Ser. No.10/952,200, filed Sep. 28, 2004 entitled Method for Controlling NitrogenFraction discloses one such system. The stack control module 62 alsocalculates the cathode airflow request for the current stack load andprovides a stack airflow request signal on line 72.

The bleed request signal is sent to an anode control module 64 thatprovides a bleed command signal on line 68 to a bleed valve in a fuelcell system 70. The anode control module 64 uses a valve model tocalculate the hydrogen that will flow through the bleed valve 26 and mixwith the cathode exhaust gas. The anode control module 64 adjusts thedifferential pressure across the bleed valve 26 to control the anodeexhaust gas flow therethrough during the bleed by controlling theopening of the valve 26. The anode control module 64 also generates anairflow request signal on line 74 that needs to be provided to dilutethe hydrogen in the anode exhaust gas during the bleed so that thepercentage of hydrogen in the mixed exhaust gas is below thepredetermined safety level.

Both of the airflow request signals on the lines 72 and 74 are sent to amaximum processor 76 that takes the larger of the two values, and sendsit to a cathode control module 78 on line 88. The cathode control module78 also receives a measured cathode airflow signal on line 80 from theflow sensor 46 indicating the actual airflow from the compressor 30. Thecathode control module 78 generates a compressor command signal that issent to the fuel cell system 70 on line 82 for controlling the speed ofthe compressor 30. The compressor command signal will satisfy both thestack load requirement and the amount of air at the mixing junction 36that is necessary to dilute the hydrogen below the predetermined safetylevel during the nitrogen bleed.

In order to assure a worst-case estimation of the hydrogen concentrationin the mixed exhaust gas, the cathode control module 78 provides aguaranteed airflow signal on line 84 that takes potential sensorinaccuracies and production tolerances into consideration. Theguaranteed airflow signal considers airflow and model accuracies,production tolerances, mass flow splits between the main airflow to thecathode inlet of the stack 12 and to the mixing junction 36, etc. Theguaranteed airflow signal on the line 84 and the airflow request signalon the line 74 are compared in the comparator 86. If the airflow requestsignal is smaller than the guaranteed airflow signal, the hydrogenconcentration limit will not be exceeded, and the bleed valve 26 can beopened. Otherwise, the anode control module 64 will not open the bleedvalve 26.

As mentioned above, the stack control module 62 can request a nitrogenbleed using various techniques. In one embodiment, the bleed valve 26 isopened if the following equation is true.

$\begin{matrix}{x_{H_{2},{Offgas},\max} > \frac{{dn}_{H_{2},{An},{Out}}}{\begin{matrix}{{dn}_{N_{2},{An},{Out}} + {dn}_{N_{2},{Cath},{Out}} + {dn}_{O_{2},{Cath},{Out}} + {dn}_{H_{2},{An},{Out}} +} \\{{dn}_{{H_{2}O},{Cath},{Out},{Vap}} + {dn}_{{H_{2}O},{An},{Out},{Vap}}}\end{matrix}}} & (1)\end{matrix}$Where:

$\begin{matrix}{{dn}_{N_{2},{Cath},{Out}} = {\frac{x_{N_{2},{Cath},{In}}*{dm}_{{Air},{Cath},{In}}}{M_{Air}} - {dn}_{N_{2},{Cath},{ToAnPermeation}}}} & (2)\end{matrix}$dn_(H) ₂ _(O,Cath,Out,Vap)=0  (3)

Where equation (3) is a worst case assumption that all of the water iscondensed.

$\begin{matrix}{{dn}_{O_{2},{Cath},{Out}} = {\frac{x_{O_{2},{Cath},{In}}*{dm}_{{Air},{Cath},{In}}}{M_{Air}} - \frac{I*n}{2*Q_{e}*N_{a}}}} & (4)\end{matrix}$Where d is a derivative with respect to time.

As mentioned above, the anode control model 64 uses a valve model tocalculate the hydrogen that is bled from the bleed valve 26. Accordingto one embodiment of the present invention, the valve model uses thefollowing equation to provide the calculation.

$\begin{matrix}{Q = {1.0219*{kv}*\sqrt{\frac{\rho_{n}*\left( {p_{1}^{2} - p_{2}^{2}} \right)}{T}}}} & (5)\end{matrix}$Where kv is the characteristic value for the bleed valve 26, Q is theflow rate of the anode exhaust gas flowing through the bleed valve 26,p₁ is the pressure at the inlet of the bleed valve 26, p₂ is thepressure at the outlet of the bleed valve 26, ρ_(n) is the density ofthe anode exhaust gas and T is the temperature of the anode exhaust gas.

Because the hydrogen concentration in the mixed anode and cathodeexhaust gas cannot be higher than the predetermined limit, theparameters and sensor signals should be chosen in a way that the anodeexhaust gas flow Q_(calculated) would be higher or equal to the realunknown anode exhaust gas flow Q_(real). This results in the followingworst case assumptions for equation (5).kv _(worst) _(—) _(case) =kv _(real) +Δkv _(tolerance)  (6)T _(worst) _(—) _(case) =T _(real) −ΔT _(tolerance)  (7)p _(1,worst) _(—) _(case) =p _(1,real) +p _(1,tolerance)  (8)p _(2,worst) _(—) _(case) =P _(2,real) +P _(2,tolerance)  (9)

The flow rate of the anode exhaust gas through the bleed valve 26 can becalculated in other ways. According to another embodiment of theinvention, the flow rate through the bleed valve 26 is calculated as:

$\begin{matrix}{{dn}_{{BleedValve},{An},{Out}} = {27.778*C_{v}*\sqrt{\frac{p_{1}^{2} - p_{2}^{2}}{T_{1}*\frac{M_{Bleed}}{M_{Air}}}}}} & (10)\end{matrix}$Due to sensor accuracy, the outlet pressure p₂ is replaced by a deltapressure Δp_(BleedValve) with:p ₂=(p ₁ −Δp _(BleedValve))²  (11)Which leads to:

$\begin{matrix}{{dn}_{{BleedValve},{An},{Out}} = {27.778*C_{v}*\sqrt{\frac{\Delta\; p_{BleedValve}*\left( {{2*p_{1}} - {\Delta\; p_{BleedValve}}} \right)}{T_{1}*\frac{M_{Blood}}{M_{Air}}}}}} & (12)\end{matrix}$Where:

$\begin{matrix}{M_{Bleed} = {{\sum\limits_{i = 1}^{3}{M_{i}*x_{i,{Bleed}}}} = {{M_{H_{2}}*x_{H_{2},{Bleed}}} + {M_{N_{2}}*x_{N_{2},{Bleed}}} + {M_{H_{2}O}*x_{{H_{2}O},{Bleed}}}}}} & (13)\end{matrix}$dn _(H) ₂ _(,An,Out) =dn _(BleedValve,An,Out) *x _(H) ₂ _(,Bleed)  (14)dn _(N) ₂ _(,An,Out) =dn _(BleedValve,An,Out) *x _(N) ₂ _(,Bleed)  (15)dn _(H) ₂ _(O,An,Out,Vap) =dn _(BleedValve,An,Out) *x _(H) ₂_(O,Bleed)  (16)All of the water fractions and flows referred to in the equations aboveare for vaporized water.

The following assumptions are made for the flow rate calculation above.x_(H) ₂ _(,Bleed)=1  (17)Equation (17) is for the worst case where the anode exhaust gas is pureH₂, but will be less than 1 if better information is available. Ifequation (17) is assumed to be 1, then equations (18) and (19) beloware:x_(N) ₂ _(,Bleed)=0  (18)If it is assumed that the hydrogen fraction is 1, then all of theotherfractions in the bled gas are assumed to be 0. Equation (18) is forthe worst case where the anode exhaust gas is pure H₂, but will be morethan 0 if better information is available.x_(H) ₂ _(O,Bleed)=0  (19)Equation (19) is for the worst case where the anode exhaust gas is pureH₂, but will be greater than 0 if better information is available.C _(v) =C _(V) _(—) _(ValveDesign) +C _(v) _(—) _(DesignTolerance)  (20)Where C_(v) is a known value.T ₁ =T _(SensorReading) −T _(SensorTolerance)  (21)Where T₁ is the temperature sensor reading and includes data sheetinformation.p ₁ =p _(SensorReading) +p _(SensorTolerance)  (22)Where p₁ is the pressure sensor reading and includes data sheetinformation.Δp _(BleedValve) =Δp _(SensorReading) +Δp _(SensorTolerance)  (23)Where Δp is the delta pressure sensor reading and includes data sheetinformation.

The various values used in the equations above are defined as:

C_(v) is the characteristic value of a valve [gal/min];

p₁ is the absolute value of the up stream pressure of the anode bleedvalve 26 [kPa];

p₂ is the absolute value of the downstream pressure of the anode bleedvalve 26 [kPa];

T₁ is the gas temperature at the anode bleed valve inlet [K];

Δp_(BleedValve) is the pressure difference over the anode bleed valve 26[kPa];

M_(Bleed) is the molar weight of the anode bleed flow [g/mol];

M_(i) is the molar weight of species i=N₂,H₂,H₂O [g/mol];

x_(i,Bleed) is the mole fraction of species i in the anode bleed gas;

dm_(Air,Cath,In) is the measured air mass flow at stack cathodeinlet—tolerance of mass flow sensor;

dn_(H) ₂ _(,An,Out) is the hydrogen flow at the stack anode outlet[mol/s];

dn_(N) ₂ _(,An,Out) is the nitrogen flow at the stack anode outlet[mol/s];

dn_(N) ₂ _(,Cath,Out) is the nitrogen flow at the stack anode outlet[mol/s];

dn_(O) ₂ _(,Cath,Out) is the oxygen flow at the stack anode outlet[mol/s];

dn_(N) ₂ _(,CathToAnPermeation) is the nitrogen flow permeating from thestack cathode side through membrane(s) into the stack anode side[mol/s];

dn_(H) ₂ _(O,Chat,Out) is the vaporized water stream at the stackcathode outlet [mol/s];

dn_(H) ₂ _(O,An,Out) is the vaporized water stream at the stack anodeoutlet [mol/s];

I is the stack current [A] and tolerance of sensor;

n is the number of cells in the stack;

Q_(e) is the elementary charge (1.6022 e-19 Coulomb);

N_(a) is the Avagadro Constant (6.022 e23);

M_(Air) is the maximum molar weight of air at all ambient conditions inwhich vehicle operation is possible [g/mol];

x_(O) ₂ _(,Cath,In) is the minimum molar fraction of oxygen in air atall ambient conditions in which vehicle operation is possible;

x_(N) ₂ _(,Cath,In) is the minimum molar fraction of nitrogen in air atall ambient conditions in which vehicle operation is possible; and

x_(H) ₂ _(,Offgas,Max) is the maximum allowed molar fraction of hydrogenin the air at end of the vehicle tailpipe.

The value dn_(N) ₂ _(,CathToAnPermeation) is dependant on the stacktemperature and the nitrogen partial pressure and can be calculated bymembrane permeation models. The value dn_(H) ₂ _(O,Cath,Out,Vap)=0 couldbe replaced by a better value if this stream is known exactly.

The desired air mass flow can be calculated as:

$\begin{matrix}{{dm}_{{Air},{Cath},{In},{des}} = {M_{Air}*\left( \frac{1}{1 - x_{{H_{2}O},{Cath},{In}}} \right)*\left( {{{dmol}_{{H2},{An},{Out}}*\left( {\frac{1}{x_{{H2},{Offgas},{Max}}} - 1} \right)} - {dn}_{N_{2},{An},{Out}} - {dn}_{{H_{2}O},{An},{Out}} - {dn}_{{H_{2}O},{Cath},{Out},{Vap}} + \frac{I*n}{2*Q_{e}*N_{a}} + {dn}_{{N2},{CathToAnPermeation}}} \right)}} & (24)\end{matrix}$Where dm_(Air,Cath,In,des) is the desired air mass flow at the cathodeinlet and x_(H) ₂ _(O,Cath,In) is the maximum molar fraction of water inair at all ambient conditions in which vehicle operation is possible.Water is not taken into account because it may condense before theexhaust gas leaves the tailpipe. If the content of vaporized water atthe end of the tailpipe is known it could be integrated into the formulato reduce the airflow command.

The foregoing discussion discloses and describes merely exemplaryembodiments of the present invention. One skilled in the art willreadily recognize from such discussion and from the accompanyingdrawings and claims that various changes, modifications and variationscan be made therein without departing from the spirit and scope of theinvention as defined in the following claims.

1. A fuel cell system comprising: a fuel cell stack including a cathodeside and an anode side; a compressor providing an airflow to the cathodeside of the fuel cell stack; a bleed valve for periodically bleedinganode exhaust gas from the anode side of the stack; a mixing junctionfor mixing a cathode exhaust gas and the bled anode exhaust gas duringthe bleed; and a controller for controlling the compressor and the bleedvalve so that the concentration of hydrogen in the mixed anode andcathode exhaust gas is below a predetermined percentage, said controllerusing a valve orifice model to calculate the flow rate of the anodeexhaust gas through the bleed valve to identify how much airflow fromthe compressor is required to dilute the hydrogen in the anode exhaustgas to be below the predetermined percentage.
 2. The system according toclaim 1 wherein the controller raises the compressor airflow to allowoperation of the bleed valve in the event that the cathode airflow tothe stack is below a predetermined value.
 3. The system according toclaim 1 further comprising a first pressure sensor for measuring thepressure of the anode exhaust gas, a second pressure sensor formeasuring the delta pressure across the bleed valve and a temperaturesensor for measuring the temperature of the anode exhaust gas, saidcontroller using the measured pressures and the temperature forcalculating the flow rate of the anode exhaust gas through the bleedvalve.
 4. The system according to claim 3 wherein the controller usessensor inaccuracies and production tolerances in the valve orifice modelto ensure that the concentration of hydrogen in the mixed anode andcathode exhaust gas is below the determined percentage.
 5. The systemaccording to claim 1 wherein the controller calculates the flow rate ofthe anode exhaust gas by the equation:$Q = {1.0219*{kv}*\sqrt{\frac{\rho_{n}*\left( {p_{1}^{2} - p_{2}^{2}} \right)}{T}}}$where kv is the characteristic value for the bleed valve, Q is the flowrate of the anode exhaust gas flowing through the bleed valve, p₁ is thepressure at the inlet of the bleed valve, p₂ is the pressure at theoutlet of the bleed valve, ρ_(n) is the density of the anode gas and Tis the temperature of the anode exhaust gas.
 6. The system according toclaim 1 wherein the controller calculates the flow rate of the anodeexhaust gas by the equation:${dn}_{{BleedValve},{An},{Out}} = {27.778*C_{v}*\sqrt{\frac{p_{1}^{2} - p_{2}^{2}}{T_{1}*\frac{M_{Bleed}}{M_{Air}}}}}$where C_(v) is the characteristic value for the bleed valve,dn_(BleedValve,An,Out) is the flow rate of the anode exhaust gas flowingthrough the bleed valve, p₁ is the pressure at the inlet of the bleedvalve, p₂ is the pressure at the outlet of the bleed valve, M_(Bleed) isthe molar weight of the anode exhaust gas, M_(Air) is the maximum molarweight of air and T is the temperature of the anode exhaust gas.
 7. Amethod for limiting the concentration of hydrogen in a mixed cathode andanode exhaust gas from a fuel cell stack, said method comprising:periodically bleeding an anode exhaust gas to be mixed with the cathodeexhaust gas from the stack; and controlling a compressor and a bleedvalve so that the concentration of hydrogen in the mixed anode andcathode exhaust gas is below a predetermined percentage, whereincontrolling a compressor and a bleed valve includes using a valveorifice model to calculate the flow rate of the anode exhaust gasthrough the bleed valve to identify how much airflow from the compressoris required to dilute the hydrogen in the anode exhaust gas to be belowthe predetermined percentage.
 8. The method according to claim 7 whereincontrolling a compressor and a bleed valve includes increasing thecompressor airflow to allow the bleed valve to be opened in the eventthat an airflow to the stack is below a predetermined value.
 9. Themethod according to claim 7 further comprising measuring the pressure ofthe anode exhaust gas, measuring the delta pressure across the bleedvalve and measuring the temperature of the anode exhaust gas, whereincontrolling a compressor and a bleed valve includes using the measuredpressures and the temperature for calculating the flow rate of the anodeexhaust gas through the bleed valve.
 10. The method according to claim 9wherein controlling a compressor and a bleed valve includes using sensorinaccuracies and production tolerances in the valve orifice model toensure that the concentration of hydrogen in the mixed anode and cathodeexhaust gas is below the determined percentage.
 11. The method accordingto claim 7 wherein controlling a compressor and a bleed valve includesusing the equation:$Q = {1.0219*{kv}*\sqrt{\frac{\rho_{n}*\left( {p_{1}^{2} - p_{2}^{2}} \right)}{T}}}$where kv is the characteristic value for the bleed valve, Q is the flowrate of the anode exhaust gas flowing through the bleed valve, p₁ is thepressure at the inlet of the bleed valve, p₂ is the pressure at theoutlet of the bleed valve, ρ_(n) is the density of the anode exhaust gasand T is the temperature of the anode exhaust gas.
 12. The methodaccording to claim 7 wherein controlling a compressor and a bleed valveincludes using the equation:${dn}_{{BleedValve},{An},{Out}} = {27.778*C_{v}*\sqrt{\frac{p_{1}^{2} - p_{2}^{2}}{T_{1}*\frac{M_{Bleed}}{M_{Air}}}}}$where C_(v) is the characteristic value for the bleed valve,dn_(BleedValve,An,Out) is the flow rate of the anode exhaust gas flowingthrough the bleed valve, p₁ is the pressure at the inlet of the bleedvalve, p₂ is the pressure at the outlet of the bleed valve, M_(Bleed) isthe molar weight of the anode exhaust gas, M_(Air) is the maximum molarweight of air and T is the temperature of the anode exhaust gas.
 13. Afuel cell system comprising: a fuel cell stack including a cathode sideand an anode side; a compressor providing an airflow to the cathode sideof the fuel cell stack; an airflow sensor receiving the airflow from thecompressor and providing a flow signal; a bleed valve for periodicallybleeding anode exhaust gas from the anode side of the stack; a mixingjunction for mixing a cathode exhaust gas and the bled anode exhaust gasduring the bleed; and a controller sub-system for controlling thecompressor and the bleed valve so that the concentration of hydrogen inthe mixed anode and cathode exhaust gas is below a predeterminedpercentage, said controller sub-system including a stack control modulefor generating a first airflow request signal to open the bleed valve,said controller sub-system further including an anode control modulereceiving the first airflow request signal and providing a bleed commandsignal to the bleed valve to open the bleed valve to provide the bleed,said anode control module providing a second airflow request signal thatidentifies how much airflow from the compressor is required to dilutethe hydrogen in the anode exhaust gas to be below the predeterminedpercentage, said controller sub-system comparing the first airflowrequest signal from the stack control module and the second airflowrequest signal from the anode control module and selecting the larger ofthe two, said controller sub-system further including a cathode controlmodule that receives the larger of the first and second airflow requestsignals and receives the flow signal of the airflow from the airflowsensor, said cathode control module providing a signal to control thespeed of the compressor to maintain the hydrogen concentration below thepredetermined level during an anode exhaust gas bleed.
 14. The systemaccording to claim 13 wherein the controller sub-system compares aguaranteed airflow signal from the cathode control module and the secondairflow request signal from the anode control module so that the anodecontrol module knows not to open the bleed valve if the guaranteedairflow signal is less than the airflow signal requested by the anodecontrol module.
 15. The system according to claim 13 further comprisinga first pressure sensor for measuring the pressure of the anode exhaustgas, a second pressure sensor for measuring the delta pressure acrossthe bleed valve and a temperature sensor for measuring the temperatureof the anode exhaust gas, said anode control module using the measuredpressures and the temperature for calculating the airflow requestsignal.
 16. The system according to claim 15 wherein the anode controlmodule considers sensor inaccuracies and production tolerances whencalculating the second airflow request signal to ensure that theconcentration of hydrogen in the mixed anode and cathode exhaust gas isbelow the predetermined percentage.
 17. The system according to claim 13wherein the anode control module uses a valve orifice model to calculatethe second airflow request signal.
 18. The system according to claim 17wherein the valve orifice model calculates the flow rate of the anodeexhaust gas through the bleed valve.
 19. The system according to claim18 wherein the flow rate of the anode exhaust gas is calculated by theequation:$Q = {1.0219*{kv}*\sqrt{\frac{\rho_{n}*\left( {p_{1}^{2} - p_{2}^{2}} \right)}{T}}}$where kv is the characteristic value for the bleed valve, Q is the flowrate of the anode gas flowing through the bleed valve, p₁ is thepressure at the inlet of the bleed valve, p₂ is the pressure at theoutlet of the bleed valve, ρ_(n) is the density of the anode exhaust gasand T is the temperature of the anode exhaust gas.
 20. The systemaccording to claim 18 wherein the flow rate of the anode exhaust gas iscalculated by the equation:${dn}_{{BleedValve},{An},{Out}} = {27.778*C_{v}*\sqrt{\frac{p_{1}^{2} - p_{2}^{2}}{T_{1}*\frac{M_{Bleed}}{M_{Air}}}}}$where C_(v) is the characteristic value for the bleed valve,dn_(BleedValve,An,Out) is the flow rate of the anode exhaust gas flowingthrough the bleed valve, p₁ is the pressure at the inlet of the bleedvalve, p₂ is the pressure at the outlet of the bleed valve, M_(Bleed) isthe molar weight of the anode exhaust gas, M_(Air) is the maximum molarweight of air and T is the temperature of the anode exhaust gas.
 21. Thesystem according to claim 13 wherein the controller sub-system raisesthe compressor airflow to allow operation of the bleed valve in theevent that the cathode airflow to the stack is below a predeterminedvalue.